Techniques for reduced jitter in digital isolators

ABSTRACT

An apparatus for communicating using an isolation channel includes a transmitter circuit having a first terminal configured to communicate a first signal. The first signal oscillates in response to a data signal having a first signal level and is constant in response to the data signal having a second signal level. The transmitter circuit includes a second terminal configured to communicate that oscillates in response to the data signal having the second signal level and is constant in response to the data signal having the first signal level. The apparatus may include a receiver circuit configured to generate a recovered data signal having a first transition in a first direction between first and second levels based on an edge of a first received signal and having a second transition in a second direction between the first and second levels based on an edge of a second received signal.

BACKGROUND

1. Field of the Invention

The invention relates to isolation technology and more particularly tocommunication across an isolation channel.

2. Description of the Related Art

In a typical control application, a processor system provides one ormore control signals for controlling a load system. During normaloperation, a large DC or transient voltage difference may exist betweenthe domain of the processor system and the domain of the load system,thus requiring an isolation barrier between the processor system and theload system. For example, one domain may be “grounded” at a voltagewhich is switching with respect to earth ground by hundreds or thousandsof volts. Accordingly, an intermediate system includes isolation thatprevents damaging currents from flowing between the processor system andthe load system. Although the isolation prevents the processor systemfrom being coupled to the load by a direct conduction path, an isolationchannel allows communication between the two systems using optical(opto-isolators), capacitive, inductive (transformers), orelectromagnetic techniques.

In general, on-off keying (OOK) is a form of amplitude-shift keyingmodulation that represents digital data as the presence or absence of acarrier wave or oscillating signal. The presence of the carrier for aparticular duration represents a binary one, while its absence for thesame duration represents a binary zero. This type of signaling is robustfor isolation in driver applications because a logic ‘0’ state sends thesame signal (e.g., nothing) as when the primary side loses power and thedevice gracefully assumes its default state. That behavior isadvantageous in driver applications because it will not accidentallyturn on a device being driven, even when the primary side loses power.However, a typical isolator receives the digital signal from theprocessor system asynchronous to a clock internal to the isolatorsystem. Thus, this technique may introduce a substantial amount ofjitter into the driver control signal. Accordingly, improved techniquesfor communicating across an isolation channel are desired.

SUMMARY OF EMBODIMENTS OF THE INVENTION

Various embodiments of an isolator system having reduced jitter aredisclosed. In at least one embodiment of the invention, an apparatus forcommunicating using an isolation channel includes a transmitter circuit.The transmitter circuit includes a first terminal configured tocommunicate a first signal. The first signal oscillates in response to adata signal having a first signal level and the first signal is constantin response to the data signal having a second signal level. Thetransmitter circuit includes a second terminal configured to communicatea second signal. The second signal oscillates in response to the datasignal having the second signal level. The second signal is constant inresponse to the data signal having the first signal level. Thetransmitter circuit may include a first signal generator configured toreceive the data signal as an enable signal, enable a first oscillatorin response to the first signal level of the data signal, and disablethe first oscillator in response to the second signal level of the datasignal to thereby generate the first signal. The transmitter circuit mayinclude a second signal generator configured to receive the data signalas a complementary enable signal, enable a second oscillator in responseto the second signal level of the data signal, and disable the secondoscillator in response to the first signal level of the data signal tothereby generate the second signal. The transmitter circuit may beconfigured to transmit the first signal using the first terminal andconfigured to transmit the second signal using the second terminal. Thefirst signal may oscillate for a duration of the data signal having thefirst signal level. The second signal may oscillate for a duration ofthe data signal having the second signal level. The second signal mayoscillate for a duration of a pulse indicating the data signal havingthe second signal level, the pulse having a duration less than aduration of the data signal having the second signal level. The firstsignal generator may include a first oscillator circuit. The secondsignal generator may include a one-shot signal generator configured togenerate a pulse in response to the data signal transitioning from thefirst value to the second value. The second signal generator may includea second oscillator circuit configured to generate a pulse of the clocksignal in response to the pulse. The apparatus may include a receivercircuit configured to generate a first received signal based on areceived representation of the first signal and configured to generate asecond received signal based on a received representation of the secondsignal. The receiver circuit may include a combiner circuit configuredto generate a recovered data signal having a first transition in a firstdirection between the first and second levels based on an edge of thefirst received signal and having a second transition in a seconddirection between the first and second levels based on an edge of thesecond received signal.

In at least one embodiment of the invention, a method of communicatingusing an isolation channel includes generating a first signal. The firstsignal oscillates in response to a data signal having a first signallevel. The first signal is constant in response to the data signalhaving a second signal level. The method includes generating a secondsignal indicative of the data signal having a second signal level. Thesecond signal oscillates in response to the data signal having thesecond signal level. The second signal is constant in response to thedata signal having the first signal level. Generating the first signalmay include enabling a first oscillator for a duration of the datasignal having the first signal level. Generating the second signal mayinclude enabling a second oscillator for a duration of the data signalhaving the second signal level. Generating the second signal may includegenerating a pulse indicating the data signal having the second signallevel and enabling a second oscillator for a duration of the pulse. Themethod may include generating a recovered data signal having a firsttransition in a first direction between the first and second levelsbased on an edge of the first received signal and having a secondtransition in a second direction between the first and second levelsbased on an edge of the second received signal. The recovered datasignal may be DC correct.

In at least one embodiment of the invention, an apparatus forcommunicating using an isolation channel includes a receiver circuitincluding a first terminal configured to receive from an isolationchannel a representation of a first signal. The representation of thefirst signal oscillates in response to a data signal having a firstsignal level. The representation of the first signal is constant inresponse to the data signal having a second signal level. The receiverincludes a second terminal configured to receive from the isolationchannel a representation of a second signal. The representation of thesecond signal oscillates in response to the data signal having thesecond signal level and the representation of the second signal isconstant in response to the data signal having the first signal level.The receiver circuit may be configured to generate a first receivedsignal based on the representation of the first signal received from thefirst terminal. The receiver circuit may be configured to generate asecond received signal based on the representation of the second signalreceived from the second terminal. The receiver circuit may include acombiner circuit configured to generate a recovered data signal having afirst transition in a first direction between the first and secondlevels based on an edge of the first received signal and having a secondtransition in a second direction between the first and second levelsbased on an edge of the second received signal. The recovered datasignal may be DC correct.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood, and its numerousobjects, features, and advantages made apparent to those skilled in theart by referencing the accompanying drawings.

FIG. 1 illustrates a functional block diagram of an exemplary digitalisolator system.

FIG. 2 illustrates a functional block diagram of an exemplary digitalisolator system.

FIG. 3 illustrates waveforms for the digital isolator system of FIG. 2.

FIG. 4 illustrates a functional block diagram of a low-jitter digitalisolator system.

FIG. 5 illustrates a circuit diagram of an exemplary oscillator of thelow-jitter digital isolator system of FIG. 4.

FIG. 6 illustrates waveforms for the low-jitter digital isolator systemof FIG. 4.

FIG. 7 illustrates a functional block diagram of a low-jitter digitalisolator system consistent with at least one embodiment of theinvention.

FIG. 8 illustrates waveforms for the low-jitter digital isolator systemof FIG. 7.

FIG. 9 illustrates a functional block diagram of a combiner of thelow-jitter digital isolator system of FIG. 7 consistent with at leastone embodiment of the invention.

FIG. 10 illustrates waveforms for combiner of FIG. 9 consistent with atleast one embodiment of the invention.

FIG. 11 illustrates a functional block diagram of a low-jitter digitalisolator system consistent with at least one embodiment of theinvention.

FIG. 12 illustrates waveforms for the low-jitter digital isolator systemof FIG. 10.

FIG. 13 illustrates a functional block diagram of a combiner of thelow-jitter digital isolator system of FIG. 11 consistent with at leastone embodiment of the invention.

FIG. 14 illustrates waveforms for the combiner of FIG. 13 consistentwith at least one embodiment of the invention.

FIG. 15 illustrates a functional block diagram of a multi-chip moduleincluding a digital isolator system consistent with at least oneembodiment of the invention.

FIG. 16 illustrates a functional block diagram of a digital isolatorsystem in an exemplary driver application.

The use of the same reference symbols in different drawings indicatessimilar or identical items.

DETAILED DESCRIPTION

Referring to FIG. 1, in an exemplary application, controller 102, whichmay be a microprocessor, microcontroller, or other suitable processingdevice, operates in a first domain (i.e., a domain including VDD1, e.g.,5 Volts (V)) and provides one or more signals (e.g., DATA) for a highpower load system operating in a second domain (i.e., a domain includingVDD5 and VDD6, e.g., 150V and −150V, respectively, for an exemplaryclass D audio application) using digital isolator 104. Digital isolator104 includes isolation channel 120, which may isolate the domains on afirst side of system 100, e.g., the primary side including VDD1 (e.g.,less than ten volts) and VDD2 (e.g., less than ten volts) and devicescoupled thereto, and a second side of system 100, e.g., the secondaryside including VDD3 and VDD4 (e.g., tens of volts) and VDD5 and VDD6(e.g., hundreds of volts) and devices coupled thereto. In the exemplaryclass D audio application, the voltage VDD3-V_(S112) is equal to aconstant voltage (e.g., 10V-30V) and the voltage VDD4-V_(S110) is equalto a constant voltage (e.g. 10V-30V), where V_(S110) and V_(S112) arethe voltage on the source node of device 110 and the voltage on thesource node of device 112, respectively.

Isolation channel 120 facilitates safe communication of a signalreceived from controller 102 in the primary side to the secondary side.The secondary side includes driver circuitry (e.g., included in receivercircuits 108), which generates an output control signal based on thesignal received from the primary side and provides a suitable drivesignal to devices 110 and 112. In an exemplary embodiment, digitalisolator 104 includes multiple integrated circuits configured as amulti-chip module in a single package. For example, digital isolator 104includes a primary side integrated circuit (e.g., transmitter circuit106) and a secondary side integrated circuit (e.g., receiver circuits108). However, other embodiments of digital isolator 104 include onereceiver circuit 108 or additional transmitter circuits 106 and/oradditional receiver circuits 108. Transmitter circuit 106 and receivercircuit 108 include terminals that may be pins of corresponding packagesincluded in a package of the multi-chip module. Those terminals may becoupled to external elements, e.g., transformers, discrete resistors,and discrete capacitors within the multi-chip module or to terminals ofthe package of the multi-chip module, and a package of controller 102.In other embodiments, transmitter circuit 106 is formed on a firstintegrated circuit die and receiver circuit 108 is formed on anotherintegrated circuit die, both of which are attached to a lead frame andinclude terminals coupled to isolation channel 120 formed on the leadframe and packaged as a single device. In still other embodiments,transmitter circuit 106 and receiver circuit 108 include terminalscoupled to isolation channel 120, all of which are formed on anintegrated circuit die. Isolation channel 120 allows safe communicationof signals from controller 102 to receiver circuits 108, which drivehigh-power drive devices 110 and 112 used to deliver power to a loadsystem. Exemplary high-power drive devices include powermetal-oxide-semiconductor field-effect transistors (MOSFETs),insulated-gate bipolar transistors (IGBTs), Gallium-Nitride (GaN)MOSFETs, Silicon-Carbide power MOSFETs, and other suitable devices ableto deliver high power signals.

In at least one embodiment of system 100, isolation channel 120 blocksDC signals and only passes AC signals. Isolation channel 120 isillustrated as including capacitive isolation, although other suitableisolation techniques may be used. Isolation channel 120 uses on-offkeying to communicate one or more digital signals (e.g., DATA) betweentransmitter circuit 106 and receiver circuit 108. Transmitter circuit106 receives DATA asynchronously to an internal clock and generates anon-off keying representation of the digital signal of DATA by providinga signal that oscillates when DATA has a first value (e.g., a highvalue, i.e., a signal level indicating a digital ‘1’) and is constantwhen DATA has a second value (e.g., a low value, i.e., a signal levelindicating a digital ‘0’). Referring to FIGS. 2 and 3, a typicaltechnique generates that representation of DATA using a logical AND ofthe carrier clock signal, CLK, with DATA. Carrier clock 107 has afrequency much greater than a frequency associated with the DATA. Bydriving a differential signal representing DATA on a capacitivelycoupled conductor of isolation channel 120, a representation of DATA isreceived by receiver circuit 108, which may amplify that representationusing a low-noise amplifier and detect the digital data of that receivedrepresentation using energy detection or other suitable technique.Receiver circuit 108 generates a recovered digital signal, RDATA, whichis a recovered version of DATA. The recovered digital signal, RDATA, mayexhibit jitter that is approximately T_(CLK)/2 since DATA can arrive attransmitter circuit 106 at any time during the period T_(CLK) of clock107. One technique for reducing the jitter on RDATA includes increasingthe frequency of clock 107. However, increasing the carrier frequencyincreases power consumption of the system, which may be undesirable insome applications.

Referring to FIGS. 4-6, in at least one embodiment of a digital isolatorsystem, a primary side device generates a representation of a datasignal to be communicated over an isolation channel improving an edge(i.e., reducing the jitter associated with a data transition) bystarting and stopping the oscillator 304 using DATA as the enablingsignal. For example, oscillator 304 is a ring oscillator that includesswitches 306 and 308. Switch 306 is closed when DATA has a high value(i.e., DATA=‘1’) and is open otherwise. Switch 308 is closed when DATAhas a low value (i.e., DATA=‘0’) and is open otherwise. The resultingsignal, CLK, is used as the representation of DATA that is transmittedacross isolation channel 120. This technique makes the rising transitionof RDATA independent of the frequency of oscillator 304 and reducesjitter on the rising edge of RDATA. However, this technique does notimprove the falling edge of RDATA, since DATA, which is asynchronous toCLK, may still transition to zero at any time during a period of theoscillating signal. Note that other controllable oscillator type may beused.

Referring to FIGS. 7-10, in at least one embodiment of a digitalisolator system, jitter is reduced on both the rising and falling edgesof RDATA using expanded isolation channel 606 that includes a channelfor communicating a representation of DATA indicative of DATA having ahigh value (e.g. signal DATA_ONE) and an additional channel forcommunicating a representation of DATA indicative of DATA having a lowvalue (e.g., signal DATA_ZERO). Signal DATA_ONE oscillates in responseto DATA having a high value and is constant otherwise, and signalDATA_ZERO oscillates in response to DATA having a low value and isconstant otherwise. Transmitter circuit 602 generates the DATA_ONE andDATA_ZERO signals using oscillators 610, which receive complementaryversions of DATA as enable signals. Circuits 612, which may includedriver circuits, transmit DATA_ONE and DATA_ZERO to expanded isolationchannel 606.

Circuits 614 of receiver circuit 604 include low-noise amplifiers,energy detectors, and other suitable receiver circuitry to receive twodifferential signals based on DATA_ONE and DATA_ZERO and generateRDATA_ONE and RDATA_ZERO based on those received differential signals.Combiner 610 combines RDATA_ONE and RDATA_ZERO to generate RDATA, whichis a recovered version of DATA. Combiner 610 generates RDATA bygenerating a first transition of RDATA (e.g., rising edge) using atransition of RDATA_ONE (e.g., rising edge) and generating a secondtransition of RDATA (e.g., falling edge) using a transition ofRDATA_ZERO (e.g., rising edge). Those transitions of RDATA_ONE andRDATA_ZERO are independent of the frequency of oscillators 610.

Referring to FIGS. 9 and 10, an exemplary combiner 610 includes D-typeflip-flop 616 that receives a clock signal, CLK, which is based ondelayed versions of RDATA_ONE and RDATA_ZERO. Falling edge delayelements 612 delay only the falling edges of an input signal (e.g.,RDATA_ONE or RDATA_ZERO) while the rising edge remains substantiallyunchanged. The clock signal is high only when RDATA_ONE_DLY andRDATA_ZERO_DLY are both high, which occurs in response to transitions ofDATA. That state is caused by the delay of the falling edge ofRDATA_ZERO_DLY and the substantially unchanged rising edge ofRDATA_ONE_DLY in response to a rising edge of DATA and by thesubstantially unchanged rising edge of RDATA_ZERO_DLY and the delayedfalling edge of RDATA_ONE_DLY. When CLK transitions from low to high,DFF 616 latches the complement of the output of DFF 616. Theasynchronous set terminal (SB) and asynchronous reset terminal (RB)guarantee that the output signal RDATA is DC correct, i.e., DFF 616outputs high RDATA in response to signals indicating a high level ofDATA (e.g., RDATA_ONE_DLY being high and R_DATA_ZERO_DLY being lowcauses an asynchronous set) and DFF 616 outputs low RDATA in response toa signal indicating a low level of DATA (e.g., RDATA_ONE_DLY being lowcauses an asynchronous reset). As referred to herein, the output of acircuit is DC correct if the output has the correct DC state in theevent of a power failure. For example, in the event of a power failure,transmitter circuit 602 stops transmitting and provides a ‘0’ or otherdefined state. If receiver circuit 604 loses power, receiver circuit 604provides a ‘0’ or other defined state. Although receiver circuit 604loses state information in response to the power failure, receivercircuit 604 quickly restores the correct output level upon power-up. Inexemplary high-power applications, the delay in DC-correctness must bevery fast (e.g., in tens to hundreds of nanoseconds) since powertransistors may be damaged if configured in the wrong state for even ashort period of time. Note that the logic of FIG. 9 is exemplary onlyand other combinations of logic devices and/or state elements may beused to generate RDATA based on RDATA_ONE and RDATA_ZERO.

FIGS. 11-14 illustrate another embodiment of a digital isolator systemthat reduces jitter on both the rising and falling edges of RDATA usingexpanded isolation channel 606 described above. Similar to transmittercircuit 602 described above, transmitter circuit 702 transmits DATA_ONE,which oscillates for the duration of DATA having a high value and isconstant otherwise. However, unlike transmitter circuit 602, whichtransmits a representation of DATA_ZERO that oscillates for the durationof DATA having a low value, transmitter circuit 702 transmits arepresentation of DATA_ZERO that includes only a pulse of the carriersignal in response to DATA having a low value and is constant otherwise.Receiver circuit 704 uses circuit 614 to receive the associateddifferential signals and uses energy detection or other suitabletechniques to generate RDATA_ONE and RDATA_ZERO.

Combiner 710 combines the signals received over the two separatechannels to recover RDATA by generating a rising edge of RDATA using arising edge of RDATA_ONE and generating a falling edge of RDATA using arising edge of RDATA_ZERO. An exemplary combiner 710 is illustrated inFIGS. 13 and 14. Note that combiner 710 includes only falling edge delayelement 714, which delays only one edge of the signal (i.e., fallingedge) and simple logic gates to generate RDATA. Combiner 710 includes nostate elements. Combiner 710 uses the rising edge of RDATA_ONE togenerate the rising edge of RDATA and generates the falling edge ofRDATA when RDATA_ZERO transitions high. Note that falling edge delayelement 714 delays only one edge of the signal (e.g., falling edge) andintroduces a delay, d1, that is less than the pulse width, d2, ofRDATA_ZERO. Since the RDATA_ZERO is only needed for timing information,the oscillator of FIG. 11 used to generate DATA_ZERO is turned off aftergenerating a pulse including that timing information. Timing of theDATA_ZERO indicates a low signal on DATA and when both DATA_ZERO andDATA_ONE are constant also indicates a low signal on DATA. By onlygenerating a pulse of the carrier signal as DATA_ZERO, the reducedjitter digital isolator of FIGS. 11-14 consumes less power than thereduced jitter digital isolator of FIGS. 7-10.

Referring to FIG. 15, in an exemplary application, reduced jitterdigital isolator 1404 includes transmitter circuit 702, expandedisolation channel 606, and receiver circuits 704. Other embodiments ofdigital isolator 1404 include one receiver circuit 704 or additionaltransmitter circuits 702 and additional receiver circuits 704.Transmitter circuit 702 and receiver circuits 704 include terminals thatmay be pins of corresponding packages included in a package of themulti-chip module. Those terminals may be coupled to external elements,e.g., transformers, discrete resistors, and discrete capacitors withinthe multi-chip module or to the terminals of the package of themulti-chip module. In other embodiments, transmitter circuit 702 isformed on a first integrated circuit die and receiver circuit 704 isformed on another integrated circuit die, both of which are attached toa lead frame and include terminals coupled to isolation channel 606formed on the lead frame and packaged as a single device. In still otherembodiments, transmitter circuit 702 and receiver circuit 704 includeterminals coupled to isolation channel 606, all of which are formed onan integrated circuit die. Isolation channel 606 allows safecommunication of signals from controller 102 to receiver circuits 704,which drive high-power drive devices 110 and 112 used to deliver powerto a load system. Referring to FIGS. 1 and 15, note that the isolationchannel required by reduced jitter digital isolator 1404 usingtechniques described above requires additional terminals for transmittercircuit 702 and receiver circuit 704, as compared to transmitter circuit106 and receiver circuit 108 included in system 104. Note that theembodiments of reduced jitter digital isolator 1404 described above mayalternatively include transmitter circuit 602 and receiver circuits 604instead of transmitter circuit 702 and receiver circuits 704,respectively.

Although reduced jitter digital isolators have been described withembodiments that are DC correct, which is required by some applications(e.g., class D audio application that drives a high-power speaker 1510,as illustrated in FIG. 16) other embodiments of reduced jitter digitalisolators consistent with the techniques described herein use detectionmechanisms that generate RDATA, but are not DC correct. In addition,RDATA_ONE and RDATA_ZERO may be generated with other polarities (e.g.,oscillating when DATA is low and constant when DATA high. Furthermore,other embodiments of reduced jitter digital isolators consistent withthe teachings herein, generate a DATA_ONE signal including only a pulseof the carrier signal in response to a rising edge of DATA and isconstant otherwise, e.g., in systems that do not require DC-correctnessor systems that otherwise ensure DC-correctness.

The description of the invention set forth herein is illustrative, andis not intended to limit the scope of the invention as set forth in thefollowing claims. For example, while the invention has been described inembodiments in which capacitive isolation channels are used, one ofskill in the art will appreciate that the teachings herein can beutilized with any isolation channel that has high-jitter data edgetransittions or that has one data edge transition that is a greatersource of jitter than the other data edge transition. In addition, whilethe invention has been described in embodiments in which differentialsignaling is used, one of skill in the art will appreciate that theteachings herein can be utilized with single-ended signaling. Variationsand modifications of the embodiments disclosed herein, may be made basedon the description set forth herein, without departing from the scopeand spirit of the invention as set forth in the following claims.

What is claimed is:
 1. An apparatus for communicating using an isolationchannel, the apparatus comprising: a transmitter circuit comprising: afirst terminal configured to communicate a first signal, the firstsignal oscillating in response to a data signal having a first signallevel and the first signal being constant in response to the data signalhaving a second signal level; and a second terminal configured tocommunicate a second signal, the second signal oscillating in responseto the data signal having the second signal level and the second signalbeing constant in response to the data signal having the first signallevel.
 2. The apparatus, as recited in claim 1, wherein the transmittercircuit further comprises: a first signal generator configured toreceive the data signal as an enable signal and enable a firstoscillator in response to the first signal level of the data signal anddisable the first oscillator in response to the second signal level ofthe data signal to thereby generate the first signal; and a secondsignal generator configured to receive the data signal as acomplementary enable signal and enable a second oscillator in responseto the second signal level of the data signal and disable the secondoscillator in response to the first signal level of the data signal tothereby generate the second signal, wherein the transmitter circuit isconfigured to transmit the first signal using the first terminal andconfigured to transmit the second signal using the second terminal. 3.The apparatus, as recited in claim 1, further comprising: the isolationchannel responsive to the first signal and the second signal to supplycorresponding representations of the first signal and the second signalacross an isolation barrier of the isolation channel.
 4. The apparatus,as recited in claim 1, wherein the first signal oscillates for aduration of the data signal having the first signal level and the secondsignal oscillates for a duration of the data signal having the secondsignal level.
 5. The apparatus, as recited in claim 1, wherein thesecond signal oscillates for a duration of a pulse indicating the datasignal having the second signal level, the duration of the pulse beingless than a duration of the data signal having the second signal level.6. The apparatus, as recited in claim 1, wherein the transmitter circuitfurther comprises: a first signal generator comprising a firstoscillator circuit; and a second signal generator comprising: a one-shotsignal generator configured to generate a pulse in response to the datasignal transitioning from the first value to the second value; and asecond oscillator circuit configured to generate a pulse of the clocksignal in response to the pulse.
 7. The apparatus, as recited in claim1, further comprising: a receiver circuit configured to generate a firstreceived signal based on a received representation of the first signaland configured to generate a second received signal based on a receivedrepresentation of the second signal, wherein the receiver circuitcomprises a combiner circuit configured to generate a recovered datasignal having a first transition in a first direction between the firstand second levels based on an edge of the first received signal andhaving a second transition in a second direction between the first andsecond levels based on an edge of the second received signal.
 8. Theapparatus, as recited in claim 1, wherein the data signal is receivedasynchronously.
 9. A method of communicating using an isolation channel,the method comprising: generating a first signal, the first signaloscillating in response to a data signal having a first signal level andthe first signal being constant in response to the data signal having asecond signal level; and generating a second signal indicative of thedata signal having a second signal level, the second signal oscillatingin response to the data signal having the second signal level and thesecond signal being constant in response to the data signal having thefirst signal level.
 10. The method, as recited in claim 9, whereingenerating the first signal comprises enabling a first oscillator for aduration of the data signal having the first signal level and generatingthe second signal comprises enabling a second oscillator for a durationof the data signal having the second signal level.
 11. The method, asrecited in claim 9, wherein generating the second signal comprisesgenerating a pulse indicating the data signal having the second signallevel and enabling a second oscillator for a duration of the pulse. 12.The method, as recited in claim 11, wherein the second signal oscillatesfor less than a duration of the data signal continuously having thesecond signal level and the second signal having a constant levelotherwise.
 13. The method, as recited in claim 9, further comprising:receiving a representation of the first signal using the isolationchannel; receiving a representation of the second signal using theisolation channel; and generating a recovered data signal having a firsttransition in a first direction between the first and second levelsbased on an edge of the first received signal and having a secondtransition in a second direction between the first and second levelsbased on an edge of the second received signal.
 14. The method, asrecited in claim 9, wherein the recovered data signal is DC correct. 15.An apparatus for communicating using an isolation channel, the apparatuscomprising: a receiver circuit comprising: a first terminal configuredto receive from an isolation channel a representation of a first signal,the representation of the first signal oscillating in response to a datasignal having a first signal level and the representation of the firstsignal being constant in response to the data signal having a secondsignal level; and a second terminal configured to receive from theisolation channel a representation of a second signal, therepresentation of the second signal oscillating in response to the datasignal having the second signal level and the representation of thesecond signal being constant in response to the data signal having thefirst signal level.
 16. The apparatus, as recited in claim 15, whereinthe receiver circuit is configured to generate a first received signalbased on the representation of the first signal received from the firstterminal, and the receiver circuit is configured to generate a secondreceived signal based on the representation of the second signalreceived from the second terminal, wherein the receiver circuit furthercomprises: a combiner circuit configured to generate a recovered datasignal having a first transition in a first direction between the firstand second levels based on an edge of the first received signal andhaving a second transition in a second direction between the first andsecond levels based on an edge of the second received signal.
 17. Theapparatus, as recited in claim 15, further comprising: the isolationchannel responsive to the first signal and the second signal to supplycorresponding representations of the first signal and the second signalacross an isolation barrier.
 18. The apparatus, as recited in claim 15,wherein the combiner circuit comprises: an edge delay circuit responsiveto the first received signal having a delayed edge and an undelayededge; an inverter circuit responsive to the second received signal; anda logic gate coupled to the delay circuit and the inverter circuit andconfigured to provide the recovered data signal having a firsttransition in a first direction between the first and second levelsbased on the undelayed edge of the first received signal and having asecond transition in a second direction between the first and secondlevels based on an edge of the second received signal.
 19. Theapparatus, as recited in claim 15, wherein the combiner circuitcomprises: a first edge delay circuit responsive to the first receivedsignal to generate a first edge-delayed signal having a first delayededge and a first undelayed edge; a second edge delay circuit responsiveto the second received signal to generate a second edge-delayed signalhaving a second delayed edge and a second undelayed edge; a stateelement coupled to receive one of the first and second edge-delayedsignals on a data port, coupled to receive a signal based on the firstand second edge-delayed signals on a clock port, and coupled to receivecontrol signals on set and reset ports based on the first and secondedge-delayed signals to thereby generate a recovered data signal havinga first transition in a first direction between the first and secondlevels based on the first undelayed edge of the first received signaland having a second transition in a second direction between the firstand second levels based on the second undelayed edge of the secondreceived signal.
 20. The apparatus, as recited in claim 15, wherein therecovered data signal is DC correct.